The present invention relates generally to a mobile telecommunication system and a channel handoff system between the mobile telecommunication systems. The present invention relates more specifically to a mobile telecommunication system and a channel handoff system between the mobile telecommunication systems intended to check deterioration of a service quality at the time of a handoff between mobile telecommunication systems provided by different vendors or operators.
In a conventional CDMA (Code Divisional Multiple Access) mobile telecommunication system under TIA (TELECOMMUNICATIONS INDUSTRY ASSOCIATION)/ET (Interim Standard)-95, a handoff to a CDMA or FDMA (Frequency Divisional Multiple Access) telecommunication system. has been performed as shown in FIG. 1.
FIG. 1 shows a schematic diagram illustrative of a conventional mobile telecommunication system.
CDMA mobile telecommunication system 100 comprises mobile telephone terminal 1 of subscriber""s own, radio base station (hereinafter referred to as BS) 10 and radio base station control unit (hereinafter, referred to as BSC) 12. BS 10 establishes a radio communication with mobile telephone terminal 1 within its own service area. BSC 12 controls the communication processing of radio BS 10 and mobile telephone terminal 1. It is assumed that mobile telephone terminal 1 has already completed an establishment of a communication with the concerned BS 10, to connect the terminal 1 with a subscriber of a PSTN (public switched telephone network), or another subscriber within the service area of the concerned base station 1. In addition to the above-described communication control, BSC 12 carries out a call channel handoff control.
It is further assumed that mobile telecommunication system 200 is a CDMA or an FDMA mobile telecommunication system which provides a telecommunication service under management of a vendor or an operator differing from that of system 100. Mobile telecommunication system 200 as well comprises mobile telephone terminal 1, BS 20 and BSC 22.
While mobile telecommunication systems 100 and 200 each have a plurality of radio base stations and radio base station control units connected to provide telecommunication services in cooperation, only one radio base station and only one radio base station control unit are represented for each of the mobile telecommunication systems in FIG. 1 for simplicity of the explanation. It is also assumed that BS 10 in mobile telecommunication system 100 and BS 20 in mobile telecommunication system 200 have overlapping service areas 34, and that each of the systems 100 and 200 has a capacity to provide a sufficiently high-quality service in the individual overlapping service area.
However, a problem comes up in the overlapping service area when neighboring mobile telecommunication systems that manage telecommunication services are provided by different vendors/operators.
It is ordinary that a radio communication interface for a frequency, for example, used in the communication between a mobile telephone terminal and a radio base station differs depending on a vendor/operator of the telecommunication system.
For this reason, mobile telephone terminal 1 in overlapping service area 34 cannot afford to establish calls at the same time between the terminal 1 itself and both of radio base stations 10 and 20 provided by different vendors/operators.
As a result, when mobile terminal 1, which has had an established communication with BS 10, is required to establish a new communication with BS 20 of a different vendor or a different operator, it is necessary both to perform a handoff in the overlapping service area and to synchronize the communication in a wireless region between mobile terminal 1 and BS 20.
Now, referring to FIG. 1, suppose there is a case where mobile terminal 1 moves in the direction from {circle around (1)} toward {circle around (6)}. While moving within the service area 14 of mobile telecommunication system 100, mobile terminal 1 maintains the established communication with base station 10 and BSC 12 through radio transmission path 10r. 
As mobile terminal 1 advances further toward {circle around (6)} through point {circle around (2)} to {circle around (3)}, it starts to enter overlapping service area 34. Mobile radio terminal 1, upon reaching a hard handoff boundary point{circle around (4)}, notices BSC 12 by means of BS 10 that the electric-field intensity of the radio signal transmitted from BC 10 of mobile telecommunication system 100 has reduced to less than a channel-drop threshold predetermined to judge whether or not the concerned call is to be dropped from mobile telecommunication system 100, as well as notices the field intensities of the radio signals transmitted from the peripheral base stations (cf. C1 and C2 in FIG. 2).
FIG. 2 is a time sequence diagram illustrative of procedures to perform a handoff of a call between the two radio telecommunication systems.
BSC 12, when receiving a notice C2, judges whether mobile telecommunication system 200 can provide a better quality of service in its coverage service area 24 to mobile telephone terminal (MT) 1 of interest than mobile telecommunication system 100 can in its own coverage service area 14, based on the information about the field intensities included in the received notice C2.
In case that BSC 12 judges the system 200 to be preferred to offer a better quality of service, then BSC 12 of mobile telecommunication system 100 sends, to BSC 22 of mobile telecommunication system 200, a request for reserving a call channel of BS 20 in order to reserve a new call channel of mobile telecommunication system 200 to be used after the hard handoff (cf. FIG. 2, C3 and C4). BSC 12 also sends, to mobile terminal 1 through BS 10, a hard-handoff signal to handoff the call to mobile telecommunication system 200 (cf. FIG. 2, C5 and C6).
Mobile terminal 1 performs a hard handoff upon receiving the hard-handoff signal and transmits a hard-handoff completion signal to BSC 22 through BS 20 (cf. FIG. 2, C7 and C8).
BSC 22, when receiving the-handoff completion signal, sends a request for releasing the concerned call channel of BS 10 to BSC 12 of mobile telecommunication system 100 so that the call channel employed before the handoff will be freed.
Mobile telephone terminal 1 thereafter is normally in an established communication with BS 20 and BSC 22 through radio transmission path 20r in coverage service area 24 of mobile telecommunication system 200.
In the case mobile telephone terminal 1 moves in the direction from {circle around (6)} toward {circle around (1)}, the terminal 1 maintains an established communication with BS 20 and BSC 22 through radio transmission path 20r while the terminal 1 stays in coverage service area 24 of mobile telecommunication system 200.
As mobile terminal 1 advances further toward {circle around (1)}, it comes to enter overlapping service area 34. Mobile radio terminal 1, upon reaching near a hard handoff boundary point {circle around (3)}, notices BSC 22 by means of BS 20 that the electric-field intensity of the radio signal transmitted from BC 20 of mobile telecommunication system 200 has reduced to less than a call-channel drop threshold predetermined to judge whether or not the concerned call is to be dropped from mobile telecommunication system 200, as well as notices the field intensities of the radio signals transmitted from the peripheral base stations (cf. C11 and C12 in FIG. 3).
BSC 22, when receiving a notice C12, judges whether mobile telecommunication system 100 can provide a better quality of service in its coverage service area 14 to mobile terminal 1 of interest than mobile telecommunication system 200 can in its own coverage service area 24, based on the information about the field intensities included in the received notice C12.
In case that BSC 22 judges the system 100 to be preferred to offer a better quality of service, then BSC 22 of mobile telecommunication system 200 sends, to BSC 12 of mobile telecommunication system 100, a request for reserving a call channel of BS 10 in order to reserve a new call channel of mobile telecommunication system 100 to be used after the hard handoff (cf. FIG. 3, C13 and C14). BSC 22 also sends, to mobile terminal 1 through BS 20, a hard-handoff signal to handoff the call to BS 10 of mobile telecommunication system 100 (cf. FIG. 3, C15 and C16).
Mobile terminal 1 performs a hard handoff upon receiving the hard-handoff signal and transmits a hard-handoff completion signal to BSC 12 through BS 10 (cf. FIG. 4, C17 and C18).
BSC 12, when receiving the handoff completion signal, sends a request for releasing the concerned call channel of BS 20 to BSC 22 in mobile telecommunication system 200 so that the call channel employed before the handoff will be freed.
Mobile telephone terminal 1 thereafter is normally in an established communication with both BS 10 and BSC 12 through radio transmission path 10r in coverage service area 14 of mobile telecommunication system 100.
A mobile telecommunication system is generally designed such that the coverage service areas of neighboring radio base stations in mobile telecommunication systems overlap each other successively to provide a mobile telephone terminal successively with a satisfactory service quality.
However, in the case different mobile telecommunication systems to which neighboring base stations belong are individually provided by different vendors/operators, the neighboring base stations often have coverage service areas with narrow overlapping regions as shown in FIG. 4, unlike the base stations having ideally wide overlapping sections as shown in FIG. 1.
Here, the term xe2x80x9cidealxe2x80x9d refers to the situation in which at a second hard-handoff boundary point, an electric field of the signal emitted by a first base station is superior in intensity to the electric field of the signal emitted by a second base station, and vice versa, where xe2x80x9ca second hard-handoff boundary pointxe2x80x9d refers to the location at which the electric field intensity of the signal emitted by the second base station falls off to a predetermined threshold (the call-channel drop threshold).
For example, referring to FIG. 1, if BS 10 and BS 20 are defined as the first base station and the second base station, respectively, then point {circle around (3)} is the second hard-handoff boundary point and point {circle around (4)} is the first hard-handoff boundary point. As is shown in FIG. 1, field intensity 10 E is higher than field intensity 20E at second hard-handoff boundary point {circle around (3)}. At the first hard-handoff boundary point {circle around (4)}, on the contrary, field intensity 20 E is higher than field intensity 10E.
According to the above-described definition, the situation represented by FIG. 1 is ideal.
In the situation represented in FIG. 4, in contrast, while 10E is higher than 20E at {circle around (3)}, 20E is not higher than 10E at {circle around (4)}. For this reason, the situation represented by FIG. 4 is not ideal.
In the non-ideal situation represented in FIG. 4, suppose the situation in which mobile telephone terminal 1, which has been supplied with a service in service area 14 of BS 10 in mobile telecommunication system 100, arrives at hard-handoff boundary point {circle around (4)}. The terminal 1 experiences an electric field as weak as the predetermined threshold value at the boundary point {circle around (4)}, so that an intervendor/interoperator hard handoff is performed from mobile telecommunication system 100 to mobile telecommunication system 200. As a result, the terminal 1 can,accept a service in service area 24 of BS 20 in mobile telecommunication system 200.
The performance of this handoff likely causes an intervendor/interoperator handoff in the reverse direction from mobile telecommunication system 200 to mobile telecommunication system 100. This is because the electric field intensity 20E that terminal 1 receives from BS 20 is weak at the time directly after the first handoff is effected from system 100 to system 200 at point {circle around (4)}.
Consequently, a problem encountered has been that the hard handoffs are repeatedly performed between the mobile telecommunication systems of the different vendors/operators, thereby degrading the service quality of the system.
Generally speaking, the mobile telecommunication systems provided by different vendors/operators have the following problem: in such mobile telecommunication systems, communication services are commonly managed using parameters defined by the individual vendors/operators. As a result, an intervendor/interoperator communication sometimes encounters an obstacle caused by inconsistent parameters determined independently by the individual vendors/operators. For example, if a call-channel add-on threshold and a call-channel drop threshold of one mobile telecommunication system are set up inconsistently with those of another system, this inconsistency will be likely to cause repetitive handoffs.
In addition, the problem of repeated hard handoffs as described above can come up even when service area 14 of BS 10 in mobile telecommunication system 100 and service area 24 of BS 20 in mobile telecommunication system 200 provide an overlapping service area large enough to guarantee a satisfactorily high service quality to be rendered to mobile telephone terminal 1.
Such repeated hard handoffs will be likely to occur when the terminal 1 makes reciprocating movements across a hard-handoff boundary point or line in overlapping service area 34 in the order of the points such as {circle around (1)} {circle around (2)} {circle around (3)} {circle around (4)} {circle around (5)} {circle around (4)} {circle around (3)} {circle around (2)} {circle around (3)} {circle around (4)} {circle around (5)}. In this case, handoffs are performed between mobile telecommunication systems 100 and 200 repeatedly every time the terminal 1 passes hard handoff boundary points {circle around (3)} and {circle around (4)}.
It is to be noted that, in actual cases, radio electric field 10E emitted by BS 10 and radio electric field 20E emitted by BS 20 cannot be represented as definite straight lines as is shown in FIG. 2, because of various variations in the radio environmental conditions. As a result, hard handoff boundary points {circle around (3)} and {circle around (4)} do not define definite points, and their locations change irregularly. Thus, even should mobile telephone terminal 1 not move, there can be an area where hard handoffs have to be repeatedly performed between mobile telecommunication systems. Such repeated handoffs give rise to a problem of degradation of service quality.
In view of the above-described problems, it is an object of the present invention to provide a mobile telecommunication system as well as a method of switching a call-channel between mobile telecommunication systems, capable of preventing the above-described repeated handoff to improve the service quality provided by the telecommunication systems.
In order to realize the above-described object, the present invention has been made allowing for the occasions that the mobile telecommunication systems provided by different vendors/operators are located adjacent so as to have their coverage service areas overlapping with each other; that the overlap area can be small so that the electric field intensity of the signal emitted by a first mobile telecommunication system at the hard handoff boundary point of the first system will be higher than the electric field intensity of the signal emitted by a second mobile telecommunication system at the same point; a radio mobile terminal can temporarily move from the service area of a first mobile telecommunication system to the service area of a second mobile telecommunication system only to return promptly to the original service area of the first system; and that radio environmental conditions can be varied irregularly.
In view of the above-described occasions, the present invention is intended to perform a handoff control only when a mobile telephone terminal is confirmed to have completed its steady entry to the service area of the destination mobile telecommunication system.
The present invention is further intended to decrease traffic on a radio interface by providing a mobile telecommunication system with means for canceling a handoff control designated along with an action time. This serves to improve the service quality of the mobile telecommunication system.
In the mobile telecommunication system according to the present invention, the radio base station control unit (BSC) has means for providing a hard-handoff command signal to command a mobile telephone terminal to execute a hard handoff. The hard-handoff command signal is prescribed to become effective at a predetermined time after the mobile telephone terminal receives the hard-handoff command signal.
The radio base station control unit also has means for providing a hard-handoff command cancellation signal to command a mobile telephone terminal to cancel said hard-handoff command signal when said hard-handoff command signal has not become effective yet.
The mobile telephone terminal has means for executing cancellation of the hard-handoff command signal in response to said hard-handoff command cancellation signal.
The telecommunication system according to the present invention comprises the mobile telecommunication systems as defined above.
In the mobile telecommunication systems, the radio base station control unit has a first means for providing a hard-handoff command signal to command a mobile telephone terminal to execute a hard handoff when the mobile telephone terminal is in the overlapped service area. The hard-handoff command signal is prescribed to become effective a predetermined time after the mobile telephone terminal receives the hard-handoff command signal. The radio base station control unit also has a second means for providing a hard-handoff command cancellation signal to command the mobile telephone terminal to cancel the hard-handoff command signal when the hard-handoff command signal has not become effective yet.
The mobile telephone terminal has a means for executing cancellation of the hard-handoff command signal in response to said hard-handoff command cancellation signal.
The second means has judging means for judging whether or not the hard handoff that has been commanded through the hard-handoff command signal and that is now deferred until the predetermined time is to be executed. In case that the judging means judges the hard-handoff not to be executed, the radio base station control unit issues the hard-handoff command cancellation signal.
It is preferred that judging means, while said hard handoff is deferred, compares an intensity of an electric field emitted by a second base station monitored by the mobile telephone terminal of interest with a predetermined call-channel-add-on threshold of the second base station and in case that the intensity of the electric field emitted by the second base station is lower than the predetermined threshold, then the radio base station control unit issues the hard-handoff command cancellation signal so that the mobile telephone terminal may remain communicating with a first base station through the current call channel. In the above description, the first base station refers to a base station that has currently a call channel established to communicate with the mobile telephone terminal of interest and the second base station refers to the base station to which a call channel is to be handed over.
A method of performing a hard handoff of a call of a mobile telephone terminal in a telecommunication system according to the present invention is executed in the above-described telecommunication system.
In the description below, a radio base station that currently establishes connection with said mobile telephone terminal through a call channel is referred to as a first base station, and a radio base station a call is to be handed over from the first base station is referred to as a second base station.
The method includes steps of:
providing a hard-handoff command signal to command the mobile telephone terminal to execute a hard handoff, the hard-handoff command signal being prescribed to become effective at a predetermined time after the mobile telephone terminal receives said hard-handoff command signal, when the mobile telephone terminal monitors an intensity of an electric field emitted by said first base station lowered below a predetermined channel-drop threshold;
judging whether or not the hard handoff that has been commanded through the hard-handoff command signal and is now deferred until said predetermined time is to be executed;
providing a hard-handoff command cancellation signal to command a mobile telephone terminal to cancel the hard-handoff command signal when the hard-handoff command signal has not become effective yet, in case that the hard-handoff is judged not to be executed;
issuing the hard-handoff command cancellation signal to the mobile telephone terminal; and
controlling said mobile telephone terminal to execute cancellation of the hard-handoff command signal in response to the hard-handoff command cancellation signal.
It is preferred that the step of judging includes steps of:
comparing an intensity of an electric field emitted by a second base station measured by the mobile telephone terminal of interest with a predetermined call-channel-add-on threshold of the second base station, while the hard handoff is deferred; and
issuing the hard-handoff command cancellation signal so that the mobile telephone terminal may remain communicating with a first base station through the current call channel, in case that the intensity of the electric field emitted by a second base station is lower than the predetermined threshold.
In the case at least two of said mobile telecommunication systems are provided by different vendors/operators, it is preferred that, when an intervendor/interoperator hard handoff is performed, a specific radio base station that currently establishes connection with said mobile telephone terminal through a call channel and that is to perform the hard handoff of interest is designated as a base station responsible for the hard-handoff.
The above and other objects, features and advantages of the present invention will become apparent from the following description referring to the accompanying drawings which illustrate an example of the preferred embodiment of the present invention.